Various types of sensors are used to determine the position of an object with respect to a reference. One type of such sensor is a short-circuit ring sensor described, for example, in German Published Patent Application DE-AS No. 23 52 851, WECKENMANN, assigned to the assignee of the present application. This publication discloses a ferromagnetic core, which may be open or closed, and is, preferably, elongated. A coil is located on the core. A short-circuit ring is movable in longitudinal direction with respect to the core. More than one ring may be provided, the ring being arranged to move with respect to the core without, for example, touching the core. It is made of electrically conductive material. The sensor operates on the basis that the inductance of the coil changes with the position of the ring. Either the ring is moved, or the core is moved, thereby determining the position of an object with respect to a reference position, in dependence on which ones of the elements--ring or core--are deemed fixed.
German Patent Disclosure Document DE-OS No. 29 24 092, ZABLER et al., describes a short-circuit ring sensor of the type previously disclosed having a double-U core, on which two short-circuit rings can move, independently of each other. To determine the position of an object, one of the rings is moved with respect to the core; the other ring is moved only slightly, and utilized for calibration purposes.
The sensors so described require comparatively complex circuits to analyze the output measuring signals; the sensors are not designed to determine the differential position of two relatively movable objects.
Automatic braking systems in automotive vehicles may require obtaining output signals representative of the position of the difference of two pistons. German Patent Disclosure Document DE-OS No. 31 24 755 and corresponding U.S. Ser. No. 382,456, filed May 24, 1982, now U.S. Pat. No. 4,462,642, describes a system which is arranged to determine the differential position of two pistons in such automotive braking systems, in which one of the pistons is operative in dependence on the deflection of a brake pedal in the vehicle, and the other piston moves in independence on the actual effective braking pressure. The relative positions of the pistons with respect to each other must be determined in order to permit control of the actual braking pressure considering the relationship of the cross sections of the two pistons. A sensor with an integrated Hall element has been proposed to effect such sensing.